- Electronics –
The Very Basics
- Charges and Fields
- Everyday Examples
- Properties of Electromagnetic Fields
The Very Basics
Electronics: at its most basic level, electronics is simply the study of electrons and how they move, why they move, how we can control their movement, and what strange and wonderful effects they cause when they do move. Everything from a simple light bulb to your mobile phone depends upon moving electrons from one place to another in a controlled fashion.
So in this course we are going to take a close look at electrons, what they are, and how and why they move around. And we will start by examining the electron at home in its natural environment – as part of an atom.
The word atom comes from ancient Greek and means ‘undivided’. This is the smallest building block of matter that can exist on its own in day to day life, and is made up of three types of subatomic particles: protons, neutrons, and electrons. If you look in the centre of the sun or inside a nuclear explosion you will find these sub-atomic particles swimming around on their own, but in everyday life you will always find them bound very tightly together into neat little packages – atoms.
The protons and neutrons form a tight cluster in the centre of the atom (called the nucleus), and the electrons orbit around the outside (a bit like the earth and other planets orbit the sun).
(This is actually a very simplified model of an atom, but it works well enough for us.)
Protons and electrons both have electrical charge, or (for short), charge.
Charge can be positive or negative. Particles which are opposite charges attract each other. They don’t have to touch. Like a magnet and a nail, when they are close to each other they pull together and the closer they are, the harder they pull. Its like they have a force field around them, and in fact they do. We call it an electric field. Just as opposite charges attract, like charges repel, or push each other away.
An electron has a charge of -1 (negatively charged) and a proton has a charge of +1 (positively charged), so protons and electrons are attracted to each other. It is this attraction that keeps the electrons tightly bound in orbit around the nucleus of the atom. (Neutrons are neutral and have no charge, and so we are not going to worry about them much.)
The number of protons and electrons in an atom determine what type of substance the atom makes up. For example, hydrogen atoms have just one proton and one electron. Copper atoms have 29 protons, 29 electrons, and 35 neutrons. Most normal atoms have the same number of protons and electrons, and so the charges balance out and there is an overall charge (or net charge) of zero.
Electrons are arranged around the nucleus in shells, a little like the layers of an onion. As atoms increase in numbers of protons and electrons, the electrons fill up from the inside shells first in a very specific order.
Here is a diagram of a Copper atom. The innermost shell can hold 2 electrons, then 8, 18, and so on. The outer shell is called the valence shell, and atoms are never happier than when it has exactly eight electrons.
(The word valence comes from the Latin word meaning powerful, and the number of electrons in this shell determines how powerful the atom is at reacting or combining with other atoms.)
Atoms are torn between two favourite states.
- They like to have the same number of protons and electrons and so have zero net charge.
- They like to have a complete set of electrons in their valence shell.
Atoms with both these conditions met are very happy and will not change things for all the tea in China. They are called the ‘Noble’ elements because they will not react with anything under normal circumstances and are very, very stable. They include Helium, Neon, Argon, and a few others, and they all have 8 valence electrons.
However, atoms with just one electron in their valence shell (or with seven) really want to get rid of that electron (or get one more) and are often very willing to put up with an unequal number of protons and electrons in order to do that. These elements are very unstable and chemically reactive, and include Hydrogen, Lithium, and Sodium (one valence electron) and Fluorine, Chlorine, and Bromine (seven valence electrons).
Most chemical reactions involve atoms ‘trading’ or ‘sharing’ electrons to fill up their outer shells. Some then end up with an unequal number of electrons and protons and so have a net positive or negative charge (we call these charged atoms ions). Because opposite charges attract, a positive ion will be attracted to a negative ion and they will stick together forming a molecule. Other atoms ‘share’ electrons and so stick together so they can do this. This fascinating world of forming partnerships between atoms to trade in electrons gives us the subject of Chemistry.
Metals have a fairly unique way of solving their ‘extra electron’ problem. Most metals have either one or two electrons in their outer shell, and would rather not have any. The atoms ‘give up’ the extra electron(s) and so become positively charged ions (they now have more protons than electrons). The ‘free’ electrons form a negatively charged ‘cloud’. The positive ions are attracted to the cloud (and vice versa) so the free electrons do not wander far from the metal ions which arrange themselves in a lattice structure with the free electrons moving freely in between.
The resulting structure is quite stable but has the unique property that the electrons can freely hop around inside the lattice. So long as there are the same number of free electrons as there are metal ions, the net charge is zero and everybody is happy.
Conductors, Insulators, and Charge Carriers
A material that allows the easy flow of electrons through it is said to conduct electricity. Metals are excellent conductors because of all the free electrons floating around in the lattice. Most other materials do not conduct electricity because the electrons are bound tightly to the atoms and molecules and cannot move around freely – these are insulators.
Normally we talk about the movement of electrons simply because that is most common, but actually we are really talking about the movement of electric charge. Electrons carry a charge of -1 and, in conductors, are fairly easy to move around. Electrons are our most common ‘charge carriers’. But we could just as well move protons around which have a charge of +1, and that too would be electricity. The snag there is that protons are usually bound so tightly in the nucleus of an atom that they are very difficult to free up. However, sometimes we can get positively or negatively charged ions to move around (remember, an ion is an atom which has lost or gained an electron and so now has a net positive or negative charge). This is why salty water conducts electricity – when salt dissolves it breaks down into sodium ions and chlorine ions, and these can move through the water carrying their charge with them. The charged particles which move the charge around are known as charge carriers, but for the remainder of this course we will be talking almost exclusively about electrons.
Charges and Fields
Electric charge, as we mentioned earlier, is a property of subatomic particles. Around a charge exists an electromagnetic field. The electromagnetic field actually consists of two separate fields, an electric field and a magnetic field. However, the two are so tightly linked that it is impossible for one to exist without the other, and hence they are often talked about as a single entity.
Because these fields are invisible and cannot be touched or felt directly it is difficult to get a concept of them. However, we can feel their effects in everyday life.
The most obvious are magnetic fields, and I’m just going to assume we are all familiar with magnets. Just as with positive and negative charges, magnets are attracted or repel each other depending on their polarity. Two north poles or two south poles push away, but opposite poles attract.
Anyone with long hair will at some point have taken off a pullover and felt crackles of static electricity and observed their hair floating around and defying gravity. This is an excellent demonstration of an electric field. What happens is that in dragging the pullover across the hair, a few electrons (which belong to the hair) got rubbed off and are left on the pullover. The hair now has too few electrons (but the same number of protons) and so has a net positive charge. The charge has an electric field around it which repels other positive charges – the hair repels other hairs and in pushing away from each other they tend to float up in the air. The force from the electric field is strong enough to lift the hair’s own weight.
Properties of Electromagnetic Fields
- Every charge is surrounded by an electric field.
- An electric field exerts a force on another electric charge (attractive or repulsive)
- When an electric field changes, a magnetic field is generated.
- When a magnetic field changes, an electric field is generated.
Now these are four very important principles and worth looking at in a bit of detail.
1. Every charge is surrounded by an electric field.
This applies whether it is a negative charge on an electron, a positive charge on an ion, or a positive charge on your hair. The field has a direction and a strength. The direction flows away from positive charges and towards negative charges. Because the field must spread out as it gets further away from the charge, it gets weaker with distance (just like the strength of light as you get further away from a bulb.
2. An electric field exerts a force on another electric charge (attractive or repulsive)
Positive charges are pushed in the direction of the electric field, and negative charges are pulled against the field direction.
The amount of force depends on how strong the field is and how many units of charge there are.
3. When an electric field changes, a magnetic field is generated.
Electric fields change when the charge that is creating them moves, such as when electrons flow through a wire. This change in electric field creates a magnetic field, but the magnetic field only exists while the electric field is changing.
4. When a magnetic field changes, an electric field is generated.
Likewise, if a magnetic field changes (either increases or decreases, or moves) then an electric field is created. This generated electric field exists only while the magnetic field is changing. The electric field that is generated can then exert force on any charges, and so moving a magnet next to a wire can generate an electric field which will push electrons along the wire. This is one way of generating electricity.
Because electrons have electric fields (rule 1), moving electrons generates magnetic fields (rule 3). Those magnetic fields, as they grow and decay, in turn generate further electric fields (rule 4), and those electric fields then exert a force on the electrons (rule 2) which affects their original movement. It can all seem rather complicated (probably because it is), but actually things settle down and balance out and the bits we are interested in are fairly simple.
We now have all the basic rules necessary to understand the whole subject of electronics! (Well, more or less.)